Field of the Invention
This invention relates generally to a system and method for reducing the frequency and impact of damaging air/hydrogen fronts in a fuel cell stack and, more particularly, to a system and method for purging the cathode and anode sides of a fuel cell stack with air while the fuel cell system is shut down after a procedure for periodically injecting hydrogen into the stack during the shut down has ended.
Discussion of the Related Art
Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell with no harmful emissions. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes, or catalyst layers, typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform reactant and humidity distribution. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a by-product of the chemical reaction taking place in the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors, or flow fields, for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
When a fuel cell system is shut down, unreacted hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen gas is able to diffuse through or cross over the membranes and react with the oxygen on the cathode side of the stack. As the hydrogen gas diffuses to the cathode side, the total pressure on the anode side of the stack is reduced. Some oxygen will be left in the cathode plumbing, and will slowly re-enter the cathode flow-field, either by convective or diffusive forces. Most of the oxygen will react with hydrogen that is locally present in the cell. Eventually, the local hydrogen will be consumed, and oxygen will start to concentrate. Eventually, oxygen will locally permeate the membrane to the anode.
When air enters the anode side of the stack it generates an air/hydrogen front that creates an elevated potential in the anode side, resulting in a lateral flow of hydrogen ions from the hydrogen flooded portion of the anode side to the air-flooded portion of the anode side. The lateral current combined with the high lateral ionic resistance of the membrane produces a significant lateral potential difference (˜0.5V) across the membrane. A locally high potential is produced between the air-filled cathode side opposite the air-filled portion of the anode side. The high potential adjacent to the electrolyte membrane causes rapid carbon corrosion, which causes the electrode carbon layer to thin. This decreases the support for the catalyst particles, which decreases the performance of the fuel cell.
In automotive applications, there are a large number of start and stop cycles required over the life of the fuel cell system, each of which may generate an air/hydrogen front as described above. Targets of 40,000 start and stop cycles would be considered reasonable. Leaving a stack in an oxygen-rich atmosphere at shut-down results in a damaging air/hydrogen event at both shut-down and start-up, where 2 to 5 μV of degradation per start and stop cycle is plausible. Thus, the total degradation over 40,000 start and stop cycle events is on the order of 100 or more mV. If the stack is left with a hydrogen/nitrogen mixture at shut-down, and the system is restarted before appreciable concentrations of oxygen have accumulated, cell corrosion during the shut-down and subsequent restart is avoided.
It is known in the art to purge the hydrogen gas out of the anode side of the fuel cell stack at system shut-down by forcing air from the compressor into the anode side at high pressure. However, the air purge still creates an air/hydrogen front that causes at least some corrosion of the carbon support, as discussed above, even though it is short in duration. Another known method in the art is to provide a cathode re-circulation to reduce carbon corrosion at system shut-down, as described in the commonly owned U.S. patent application Ser. No. 11/463,622, titled, “Method for Mitigating Cell Degradation Due to Startup and Shutdown Via Cathode Re-Circulation Combined with Electrical Shorting of Stack,” filed Aug. 10, 2006, assigned to the assignee of this application and herein incorporated by reference. Particularly, it is known to pump a mixture of air and a small amount of hydrogen through the cathode side of the stack at system shut-down so that the hydrogen and oxygen combine in the cathode side to reduce the amount of oxygen, and thus the potential that causes carbon corrosion.
It is also known to stop the cathode air flow while maintaining positive anode side hydrogen pressure at shut-down, and to then electrically short the stack to allow the oxygen to be consumed by hydrogen, followed by closing the inlet and outlet valves of the anode and cathode sides, as described in the commonly owned U.S. patent application Ser. No. 11/612,120, titled, “Method of Mitigating Fuel Cell Degradation Due to Startup and Shutdown Via Hydrogen/Nitrogen Storage,” filed Dec. 18, 2006, assigned to the assignee of this application and herein incorporated by reference.
It has been proposed in the art to reduce the frequency of the air/hydrogen fronts discussed above by periodically injecting hydrogen into the anode side of a fuel cell stack after the stack has been shut-down. For example, U.S. patent application Ser. No. 12/636,318, filed Dec. 11, 2009, titled Fuel Cell Operation Methods for Hydrogen Addition After Shutdown, assigned to the assignee of this application and herein incorporated by reference, discloses such a method for injecting hydrogen into the anode side of a fuel cell stack during system shut-down. However, at some point, the hydrogen injection process needs to be stopped at which time air will begin to diffuse into the stack. It is necessary to terminate the hydrogen sustaining technique at some point to conserve hydrogen or low voltage battery power for an extended vehicle off times. For these situations, the slow diffusion of oxygen back into the stack causes the catalytic corrosion discussed above. Therefore, it is known to provide cell-to-cell shorting positive temperature coefficient resistors that limit the voltage and electrode damage during the air diffusion. However, to reduce systems costs, it is desirable to eliminate the cell-to-cell resistors because of their expense.